NotTaR of small Gasoline Engines and Rotary Lawn Mowers : Importance of having the correct valve c..  
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Importance of having the correct valve clearance

(From: Derrick Setchell (D.Setchell@eastman.ucl.ac.uk).)

My father-in-law owned a Flymo with a 3.5 HP Briggs & Stratton. He'd had it for nearly twenty years, but never used it much because it was "hard to start", especially when hot. He has other mowers and neither of us had found time to investigate it. Why he didn't return it for warranty investigation at the start is lost in the mists of time.

This year, he felt it was taking up space in his barn and was going to throw it away, so it was rescued by yours truly. I thought the motor might be a useful standby in case the even older Aspera (European Tecumseh) on my own rotary packed up. That's been a fine engine, by the way. The B&S was in a pretty good state considering, but sure enough took up to fifteen pulls to start when cold and would not re-start at all if fully warmed up. It did look like it'd been HOT, with the usual blockage of ventilation spaces by clippings, but otherwise well preserved.

I gave it a complete carb and magneto overhaul which improved cold starting somewhat, but not the hot. I also felt it was a bit "gutless", so then investigated the combustion chamber and valves. This poor machine had virtually NO valve clearance when cold, on either lifter. The valves were still in pretty good order, so my assumption is that it was shipped that way from the factory. Once it got hot, the compression must have been zero, though it didn't obviously seem to be.

A thorough clean, careful adjustment of the valve stems with a diamond hone (out of the engine !! ) to provide the upper limit of the specified cold clearances and back together it went. What a contrast ! It now starts first pull, hot or cold, and runs like it should have done when it left the factory. My guess is the magneto and carb were fine all along in this lightly-used engine. Next time I'm going to look at those valve clearances earlier in the sequence, even if the engine isn't old....

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